Lonely Planets in Deep Space

What do other planetary systems look like? We have seen some where massive Jupiter-sized worlds orbit closer to their star than Mercury does to the Sun, baking them with radiation. Others have had multiple rocky planets within the Earth’s orbit distance. Some have planets similar to Earth in a variety of locations. But what about far away from the star? We never expected to find gas giants like Uranus and Neptune in the far reaches of our solar system. Are there planetary systems where planets live even farther away? Maybe there are planets that live in the empty darkness between stars, barely bound by gravity, wandering through the galaxy.

These are two direct images of the cometary dust and exoplanet surrounding the young star HD 106906. Credit: Paul Kalas, UC Berkeley

How is it possible that planets could be so far from a star, when they need the violent environment and gravity of the protostellar disk to form in the first place? The answer is in the gravitational interactions.

In the image above, we see the close up of a star called HD 106906. In the Gemini planet imager we see a disk of material, but no planet. However, in the wider field image taken by the Hubble Space Telescope (HST), we see a planet off to the right that is part of the system. It is a massive planet 11 times the mass of Jupiter and a distance of 650 AU (650 x Earth-Sun distance), putting it 16 times as far away as Pluto is from the Earth. How did it get there?

In the turbulent beginnings of the life of a planetary system, debris left over from the formation of the main star collects into planets, whose gravity dominates and grabs on to whatever material is left over. In interactions where there is a near miss, protoplanets, comets, asteroids, and any other debris can be sent flying far from the star, even liberating these objects from the star’s gravity and sending them off into interstellar space. It is currently unknown how many planet-size chunks of rock reside in interstellar space, but by studying how planetary systems form, we can constrain the possibilities.

Studying systems like that of HD 106906, we hope to determine how such a massive planet ended up so far away from it’s host star. There may be ancient planets that were once part of our own solar system lurking far beyond Neptune and the Kuiper Belt. Finding them would be a necessary step in determining exactly how our own solar system formed and developed the conditions for life to evolve.

The more I learn about our the origins of planetary systems and of our own solar system, the more I see how the Earth and the planets that orbit Sol are not unique, not special, and maybe not even that rare. Knowing this, it makes the possibility of life beyond Earth incredibly likely. If there is life, are we the extremely rare species that evolved intelligence? Is the universe full of microbial or unevolved life? Or are we part of a majority of species, intelligent but isolated? If this is the case, I wonder about the advanced civilizations that may exist? If they do, why haven’t we found evidence of their existence?

There are still many questions to answer about the universe, but it is my hope that our species is able to solve the problems facing our planet so that we can continue to ask these questions and more, and eventually find some answers too.